It is well known in the art of fuel delivery systems for aircraft gas turbine engines to use a positive displacement pump, such as a vane or gear pump, to pressurize fuel prior to its metering and subsequent delivery to the engine. Positive displacement pumps provide a flow whose volume is a function of the speed at which the pump is rotating. This characteristic makes it impossible to provide metered flow to the engine solely by controlling the area of a metering valve when the pump is driven by the engine. Therefore a pressure regulating valve (PRV) and a flow bypass system are used to facilitate the delivery of an accurately measured fuel flow to the engine.
The positive displacement pump is sized to ensure an excess flow capacity at all possible operating conditions. The pressurized fuel is then typically input to the PRV which operates to provide a fixed pressure drop across a metering valve controlled by an electronic engine control (EEC). Operation of the PRV and metering valve together is based upon incompressible flow theory which states that flow through a valve is a function of the area of the valve opening multiplied by the square root of the product of the pressure drop across the valve multiplied by the specific gravity of the fluid. Thus, with the PRV providing a fixed pressure drop across the metering valve, the EEC can control the area of the metering valve to provide the required fuel flow to the engine.
To maintain the fixed pressure drop across the metering valve the PRV uses a fuel bypass system from the outlet of the pump back to the inlet of the pump.
In addition, positive displacement pumps typically require a fluid to be supplied to the inlet of the pump at sufficient pressure to prevent cavitation or starvation of the pump. It is known in the art to use an electric pump at startup or lightoff to provide an initial flow of fuel from the fuel tank, at sufficient pressure to the positive displacement pump.
In order to improve aircraft fuel efficiency, typical fuel delivery systems also use an ejector pump to provide this pressurization of fuel in flight. The ejector pump operates on an excess flow of fuel bypassed by a second valve know as a motive flow valve. Typically the motive flow valve is controlled by the EEC and allows fuel to be bypassed to the ejector pump when the engine has reached a predetermined speed.
However, problems arise in that the motive flow valve is turned on independent of the PRV which is controlling the pressure drop across the metering valve. If the motive flow valve is opened when there is insufficient flow to bypass to the inlet of the positive displacement pump through the PRV the positive displacement pump could be starved causing an interruption in the supply of fuel to the engine. An interruption of 50 ms can be sufficient to cause a flameout.
A separate motive flow valve also complicates the design of the fuel delivery system in that the PRV must be designed to handle transients in the flow of fuel to the metering valve introduced when the motive flow valve is opened.